1. Field of the Invention
This invention relates to a radio terminal equipment, and more particularly to communication in a radio ad hoc network. A packet is transmitted and received in accordance with a connectionless system to perform communication between a plurality of terminals each having a radio function.
2. Description of the Related Art
A LAN is constructed from a radio ad hoc network from the points of view that equipments can be constructed readily, that a terminal equipment can be moved for communication, and so forth. The radio ad hoc network performs communication between a plurality of terminals each having a radio function by transmitting and receiving a packet in accordance with a connectionless system. Particularly, the radio ad hoc network is likely to cause a disconnection of a radio route or the like by a movement of a terminal, an obstacle, or the like. A stabilized communication quality is required with regard to multimedia communication of sound, moving pictures, and so forth.
In the radio ad hoc network, a route to a radio terminal equipment (hereinafter referred to simply as terminal) of a transmission destination is set in accordance with a routing protocol, and a packet is delivered to the terminal of the transmission destination past a relay terminal or terminals on the route. Various protocols such as the AODV and the OSR are provided as a routing protocol for a radio ad hoc network. The routing protocol for a radio ad hoc network updates route information of the network after every fixed interval of time. Usually, a system is used wherein route information is communicated between terminals and, if a new route is found, then route information for a next terminal is updated. If the updating interval for route information decreases, then communication of route information increases as much and the network traffic increases. Therefore, the updating interval is usually designed so as to be several seconds to several tens seconds.
In a conventional radio ad hock network, when a terminal moves or when route information cannot be received over an updating interval because of increase of the traffic, it sometimes occurs that the route is cut and a fixed period of time is required until a new communication route is formed. In such an instance, packets while the communication route is cut are deleted.
For example, systems have been proposed as follows. In Japanese Patent Laid-open No. 2001-127797, even if the number of relay terminals to a destination terminal increases, the amount of information to be transmitted with a packet is reduced to decrease the time for re-establishing a route when the route is cut. In Japanese Patent Laid-open No. 2000-252992, the number of control packets for re-establishment of a route is reduced to prevent reduction of the data traffic. However, the systems mentioned decrease control packets to be transmitted and received for route alteration or for control of such alteration, and have a problem in that packets relating to the route are deleted before a route is re-established.
Further, in Japanese patent Laid-open No. 11-289349, if a destination terminal moves until the route is cut and packets do not reach the destination terminal any more, then packets are retained once into a different terminal. After the destination terminal appears within a radio communicatable range of a relay terminal, the destination terminal receives the missing packets retained by the retaining terminal from the relay terminal. However, according to this system, time is required after an originating terminal transmits a packet until the packet reaches a destination terminal through the retaining terminal, and the system is not effective with multi-medial communication for which the real time property of sound, moving pictures, and so forth is required.
In a radio ad hock network, when a route relating to a terminal intermediately of a route disappears, some time is required before a new route is established. Packets relating to the route, which are included in the time period, are lost. While the real time property must be assured in multimedia communication, the packets lost within the time cannot be assured. Therefore, this makes a cause of deteriorating the quality of the service. Although systems for reduction of the time for re-establishment of a route and for reduction of control packets have conventionally been proposed, the systems do not disclose a system for preventing the loss of packets in multimedia communication.